Reiss to Reformation clothing size

The short answer

Reiss 8 is Reformation XS by the two brands' own charts.

Both brands publish the body bust each size is cut for, so every row matches a bust, not a letter or a number.

Every Reiss size read across to Reformation. UK sizes on the left, Letter on the right.
Reiss	Reiss builds it for	Reformation, by chart	Same label	Reformation builds it for	Reformation also prints
4	bust 31 in, waist 24 in, hip 34 in	not made	XS	not published	not published
6	bust 32 in, waist 25 in, hip 35 in	XS	XS	bust 33 in, waist 25 in, hip 36 in	US 0
8	bust 33 in, waist 26 in, hip 36 in	XS	S	bust 33 in, waist 25 in, hip 36 in	US 0
10	bust 35 in, waist 28 in, hip 38 in	S	S	bust 35 in, waist 27 in, hip 38 in	US 2/4
12	bust 37 in, waist 30 in, hip 40 in	M	M	bust 37 in, waist 29 in, hip 40 in	US 6/8
14	bust 39 in, waist 31.5 in, hip 42 in	L	M	bust 39.5 in, waist 31.5 in, hip 42.5 in	US 10
16	bust 41 in, waist 33.5 in, hip 44 in	XL	L	bust 41.5 in, waist 33.5 in, hip 44.5 in	US 12
18	bust 43 in, waist 36 in, hip 46.5 in	XL or 1X	L	bust 41.5 in, waist 33.5 in, hip 44.5 in	US 12

Tinted rows change label or sit between two sizes. 1 sizes have no counterpart on the Reformation chart. Read from the two charts only; no formula and no averaging.

What wearers say

Charts describe a garment, not a fit. We do not collect fit reports for this category yet, so this table is the two brands' own claims and nothing more.

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Sources

Reiss size guide, read on 2026-09-18.
Reformation size guide, read on 2026-09-18. Reformation on TrueToSize

Reformation to Reiss